1. Field of the Invention
The invention relates to an arrangement for actuating an element in a projection exposure apparatus.
2. Prior Art
Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (=reticle) illuminated via of the illumination device is in this case projected via the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In a projection exposure apparatus designed for EUV (i.e. for electromagnetic radiation having a wavelength of less than 15 nm), for lack of light-transmissive materials being available, mirrors are used as optical components for the imaging process. The mirrors can be fixed on a carrying frame (also designated as “force absorbing frame” or “force frame”) and can be designed to be at least partly manipulatable in order to enable the respective mirror to be moved for example in six degrees of freedom (i.e. with regard to displacements in the three spatial directions x, y and z and with regard to rotations Rx, Ry and Rz about corresponding axes), as a result of which it is possible, for instance, to compensate for changes in the optical properties that occur during the operation of the projection exposure apparatus, e.g. on account of thermal influences. Furthermore, in addition to the carrying frame, a sensor frame mechanically decoupled from the latter can be provided.
In this case, the respective mirror position relative to the sensor frame can be measured via a position sensor and can be set to the desired value via a controller via an actuator. In this case, during the operation of the projection exposure apparatus, the problem then occurs in principle that every force exerted on an element such as the respective mirror, for example, by an actuator, on the basis of the Newtonian principle “actio=reactio”, is accompanied by a reaction force of equal magnitude acting in the opposite direction. Action of the reaction force on the sensor frame during the operation of the projection exposure apparatus would have the consequence, however, of the sensors provided on the sensor frame measuring substantially only parasitic dynamics, and should therefore be prevented.
Known approaches for overcoming this problem include the use of a mechanical filter in the form of a spring-mass system on the force path between mirror and sensor frame. With regard to the prior art, reference is made to U.S. Pat. No. 6,788,386 B2, for example.
FIG. 5 illustrates merely schematically a typical conventional construction, wherein the mirror is designated by “10”, the sensor frame is designated by “20” and the carrying frame is designated by “30”. The actuator A, which is driven via a controller (not depicted in FIG. 5) in accordance with the signal supplied by the position sensor P for the actuation of the mirror 10, can be coupled to the carrying frame 30 via a mechanical filter in the form of a spring-mass system composed of a mass 15 and a spring 16.
In the case of mechanical filters, the suppression of the actuator forces depends on the relative distance between the excitation frequency and the filter frequency. In this case, the filter frequency fF is given by
                              f          F                =                              1                          2              ⁢              π                                ·                                    k              m                                                          (        1        )            where k denotes the spring stiffness of the spring-mass system forming the mechanical filter and m denotes the filter mass of the mechanical filter.
An exemplary transfer function of the reaction force suppression (i.e. “actuator force” with respect to “force on the carrying structure”) is shown in FIG. 4, wherein the frequency (in arbitrary units) is plotted logarithmically on the horizontal axis. Excitations in the frequency range below the filter frequency fF are not suppressed. Above the filter frequency fF, the actuator forces are increasingly suppressed with a gradient of −40dB/decade. Once the filter frequency has been chosen, then the static deflections (in the region of the stiffness line) and the dynamic deflections for a given actuator force spectrum are dependent on the absolute value of the filter mass chosen.
This has the consequence e.g. for actuators that have to carry relatively large static loads that large to impracticable (>1 cm) static deflections can occur especially at low filter frequencies (<100 Hz) on account of the small spring stiffnesses. That could admittedly be counteracted, in principle, by using large reaction masses, but that often cannot be realized on account of structural space limitations. Consequently, in order to suppress the reaction forces that occur, comparatively large filter masses are required, the integration of which into an EUV system with limited structural space available is problematic or even impossible. This increasingly applies to high-aperture EUV systems (for example EUV systems having a numerical aperture NA of greater than 0.3), in which generally the arrangement of the mirrors themselves without disturbing the optical beam path already constitutes a demanding challenge.